1. Field of the Invention
The present invention relates to a scanner using a linear CCD sensor. More particularly, the invention relates to a scanner which is adjusted in feed amount to stabilize a quantity of light during scan.
2. Related Background Art
Scanners are widely used these days as means for inputting document or graphic data into a computer, or as input means for digital copier or facsimile device.
A scanner is operated in such a manner that strong light from a light source is let to impinge on a surface of document to be read and that reflected light from the document is guided through an optical system to form an image on an image sensor. The image sensor reads the image while photoelectrically converting an optical signal into an electric signal of a voltage level proportional to the intensity of reflected light, that is, the density of document in the unit of pixel. The electric signal is amplified and subjected to analog-to-digital conversion to obtain digital data. The digital data is transferred to a host system.
A most popularly used image sensor is a linear CCD sensor in which pixels are aligned on a line, which can read image information while scanning the document by a mechanical unit.
FIG. 5 is a block diagram to show a CCD sensor used in scanner. In FIG. 5, S.sub.1, S.sub.2, . . . , S.sub.N designate light receiving elements (photodiodes), SR.sub.1, SR.sub.2, . . . , SR.sub.N analog shift registers (CCD) for shifting out analog outputs from the light receiving elements, and BUFFER an output buffer. Also, SH represents a start pulse for starting a shift operation of the shift registers, o.sub.1, o.sub.2 shift register transfer clocks, o.sub.R a reset pulse, and CCDOUT a CCD output.
FIG. 6 is a timing chart to show timings of the drive signals SH, o.sub.1, o.sub.R to the CCD sensor and CCDOUT as CCD output. Voltages generated in the light receiving elements S.sub.1, S.sub.2, . . . , S.sub.N are transferred to the analog shift registers SR.sub.1, SR.sub.2, . . . , SR.sub.N, respectively, and are successively shifted in synchronism with the transfer clocks o.sub.1, o.sub.2 to be output one by one from an output terminal of the shift registers.
FIG. 2 shows a general structure of scanner. The scanner as shown is of a reflection read-transmission read changeover type.
The scanner is provided with a scattering plate 1 on which a document (not shown) is placed, a glass table 2 for holding the scattering plate 1, a light source 3 located underneath the glass table 2, a transmission reading (TR) optical unit 4 located over the scattering plate 1, a TR optical unit sensor 8 for determining a reference position of the TR optical unit 4, a reflection reading (RR) optical unit located underneath the scattering plate 1, and a RR optical unit sensor 13 for determining a reference position of the RR optical unit 9. The TR optical unit 4 has a mirror 5, a lens and a CCD sensor 7. Also, the RR optical unit 9 has a mirror 10, a lens 11, and a CCD sensor 12.
In transmission reading, light emitted from the light source 8 passes through the glass table 2 and the scattering plate 1 to illuminate the document. The light passing through the document is reflected by the mirror 5. The light reflected by the mirror 5 is condensed by the lens 8 to impinge on a light-receptive surface of the CCD sensor 7.
In reflection reading, the scattering plate 1 is removed and the light emitted from the light source 3 passes through the glass table 2 to illuminate the document. The light reflected by the document again passes through the glass table 2 and is then reflected by the mirror 10. The light reflected by the mirror 10 is condensed by the lens 11 to impinge on a light-receptive surface of the CCD sensor 12.
The TR optical unit 4 and the RR optical unit 9 each are driven to move by an unrepresented feeder in the direction represented by the arrow A in FIG. 2. The light source 3 and the RR optical unit 9 are driven by a single driving system, and the TR optical unit 4 by another driving system.
Immediately after reading start, the light source 3 and the TR optical unit 4 are first set at the respective reference positions through the RR optical unit sensor 13 and the TR optical unit sensor 8, respectively. FIG. 2 shows a state in which the light source 3 and the TR optical unit 4 are then moved a little by moving amounts a1 and a2, respectively. Since absolute amounts are still small for the moving amounts a1 and a2 from the reference positions, errors in moving amounts are also small due to variations in driving systems, whereby the relative positional relation is kept accurate between the light source 3 and the TR optical unit 4.
Meantime, as the reading position approaches the document edge, as shown in FIG. 3, the moving amounts b1 and b2 from the reference positions measured by the sensors increase their absolute values so as to increase an error .vertline.b1-b2.vertline. due to the variations in driving systems. The reason is as follows. The TR optical unit and the light source are independently driven by a same pulse number by the separate stepping motors. Thus, they can be accurately driven by the stepping motors. However, the driving systems (pulleys, gears, belt, wire, etc.) connected to the stepping motors do have variations. The variations increase their influence on the moving amounts as the moving amounts increase.
With such a change in relative positional relation between the light source 3 and the TR optical unit 4 as shown from FIG. 2 to FIG. 3, a quantity of light reaching the CCD sensor 7 varies even if the light source 3 emits a same quantity of light.
FIG. 9 shows a state of intensity distribution of scattered light emergent from the scattering plate 1 after the light emitted from the light source 3 is scattered by the scattering plate 1. It is shown in FIG. 9 that the light is widely scattered by the scattering plate 1. The intensity of the scattered light little changes at a point 1/8 mm away from a point of the maximum intensity, or at a point 1/12 mm therefrom, but a change in quantity of light reaches 10% or more at a point 2 mm away from the point of maximum intensity.
In FIG. 2, the light emitted from the light source 3 passes through the glass table 2 and is then scattered by the scattering plate 1. Most of scattered light is reflected by the mirror 5 and passes through the lens 8 to reach the CCD sensor 7. A quantity of light reaching the CCD sensor 7 is converted into digital image data. In FIG. 3, the light emitted from the light source 3 passes through the glass table 2 and is then scattered by the scattering plate 1 similarly as in FIG. 2. Since a lot of scattered light is lost on this occasion, only a little scattered light is reflected by the mirror 5 and passes through the lens 6 to reach the CCD sensor 7. Accordingly, the quantity of light is greatly decreased at the reading end as compared with that at the reading start.
When the relative positional relation between the light source 3 and the TR optical unit 4 changes for example from FIG. 2 to FIG. 3 as described, the light quantity reaching the CCD sensor 7 also varies, which raises a problem that the brightness of image changes even upon reading a document of uniform density. This problem is serious in a color scanner in which a plurality of fluorescent tubes are juxtaposed, because changing quantities of light from the fluorescent tubes are different from each other. This could cause abnormality in color balance.